Enteric bacterial infections remain one of the greatest public health challenges worldwide. An estimated four to six million deaths per year worldwide are caused by bacterial-induced diarrhea and the associated dehydration represents the second most common cause of infant mortality around the world. Enteric bacterial infections and diarrhea are also an important public health concern for travelers and US Forces overseas. The goals of this proposal are to interrogate the influence of innate lymphoid cells (ILCs) in the regulation of innate and adaptive immunity following enteric bacterial infection. Employing Citrobacter rodentium, an experimental model of human enteropathogenic and enterohaemorrhagic Escherichia coli (EPEC or EHEC) infection, our preliminary studies identified a role for IL-23-dependent IL-22 in innate immunity to infection. The dominant cellular source of IL-23-dependent IL-22 in Citrobacter-infected mice was a population of ROR3t-positive ILCs that expressed c-kit and CD90, and in vivo depletion of ILCs in Citrobacter-infected Rag-/- mice was associated with a defect in protective innate immunity to infection. ILCs expressed the thymic stromal lymphopoietin receptor (TSLPR) and deletion of TSLPR resulted in exaggerated ILC responses and enhanced immunity to Citrobacter, indicating that TSLP inhibits ILC-dependent innate immunity in vivo. In addition, a sub-population of ILCs expressed MHC class II and costimulatory molecules and could promote T cell proliferation in vitro, suggesting that these cells also have the capacity to promote T cell-dependent immunity in vivo. Employing selective in vivo depletions, adoptive transfers and cell lineage-specific deletion strategies, three specific aims of this project will determine (i) what signals promote ILC responses in vivo and what is their role in innate immunity to Citrobacter, (ii) how TSLP-TSLPR interactions inhibit ILC responses and innate immunity, and (iii) how ILC-intrinsic MHC class II expression influences CD4pos T cell responses and immunity to Citrobacter. The results of these studies will provide a framework to test the therapeutic potential of manipulating ILC responses in the promotion of anti-bacterial immunity and the treatment of intestinal inflammation. We anticipate that defining the contribution of ILCs to anti-bacterial immunity will direct future clinical efforts to boost innate immunity in the context of human conditions involving immunodeficiency including HIV, cancer and transplantation, to improve the efficacy of oral vaccines, and to dampen ILC responses and reduce chronic inflammation in diseases such as inflammatory bowel disease.